1. Boot up HENkaku and push "select" to start the FTP Server2. On your PC open up RedSquirrels PSVita DB Theme Installer 360 Edition and enter your PSVitas IP and Port to connect.3. Open a Theme Folder to install4. Press Install Custom Theme. It will now write the required lines into the app.db and copy the Themes to ux0:customthemes. Don't cut the wifi connection while installing or you will be left with a non-working Theme.5. When you're done installing the Themes just close RedSquirrels PSVita DB Theme Installer 360 Edition and MolecuarShell on your Vita and reboot the Vita. Congrats you should now have your new Themes installed

Old Method with RedSquirrels PSVita DB Theme Installer 2.0

1. Boot up HENkaku and push "select" to start the FTP Server2. On your PC connect to your PSVita with your Preferred FTP client. I used Filezilla for this.3. Go to /ur0:/shell/db/ and download the app.db to your PC. Don't close Filezilla just yet, you will need it later.4. Copy the Themes you want to add to ux0:/theme/ (this is the folder where original themes get installed too, you can also use every other folder) Add the Themes you want with RedSquirrels Theme Tool or by Hand with SQLiteBrowser5. Delete your current app.db and then upload your modified app.db (Somehow it didn't work for me to just replace the app.db so I used this way)6. Close the connection and restart your Vita. Congrats you should now have your new Custom Themes in your Settings

I followed the guide exactly a couple of times, but I had broken images in the theme preview and the theme would not apply. The only fix I found was recreating the database from safe mode, then editing the new app.db. Just putting this out here in case anyone else has the same issue. You'll lose your custom icon placement, but it appears to fix the issue permanently. I was able to add 5 new themes without any issues.